Selectivity
Understanding Selectivity
Selectivity is quantified by the shape factor, defined as the ratio of bandwidth at a high attenuation level (e.g., -60 dB) to bandwidth at the -3 dB level. An ideal rectangular filter has a shape factor of 1:1, while practical filters range from 1.5:1 to 10:1 depending on design.
Higher selectivity requires more filter poles, higher-Q resonators, and precise tuning. Crystal, SAW, and cavity filters achieve excellent selectivity, while microstrip filters are limited by lower resonator Q values.
Frequently Asked Questions
What is shape factor?
Shape factor is the ratio of filter bandwidth at two different attenuation levels (e.g., BW at -60 dB vs BW at -3 dB). Values closer to 1.0 indicate better selectivity.
How is selectivity improved?
Adding more filter poles, using higher-Q resonator technologies, and using elliptic or quasi-elliptic topologies with transmission zeros improve selectivity.